Hi Tess,
We've already responded via email and verified that everything's working properly, but if anyone else has a link reported to have expired early it can be investigated within the sales records available at Seller Admin > View/Download Transactions Log.
Each sale will keep a record of how many download attempts have been made against a limit. Each time a link is expired it is always because the total allowed download attempts have been made or that the allowed time limit has passed.